feat: let connection pool watch for connection close (n0-computer#140)

## Description

Let the per-connection actor watch for connection close. When the
connection is closed the conn actor will tell the main actor to remove
the per-conn actor, and the next connection attempt will get a new
per-connection actor.

Also add an optional callback to wait for a connection to reach a
certain state before handing it to the user. Implements
n0-computer#141

## Breaking Changes

util::connection_pool::Config is no longer Copy
util::connection_pool::Config has a new field on_connected
util::connection_pool::PoolConnectError has a new variant OnConnectError

## Notes & open questions

Note: there is a very low probability that a request to get a
ConnectionRef will return the closed connection. But that is fine. If
the connection is closed, all currently live ConnectionRefs will turn
unusable anyway, what's one more? In connected state the actor will be
very fast in handing out ConnectionRef, so the queue should never be
full. The queue is mostly for waiting during connect.

## Change checklist

- [ ] Self-review.
- [ ] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.

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Co-authored-by: Philipp Krüger <[email protected]>

Author: rklaehn